Harry Styles and Liam Payne feared they may have caught the disease after visiting a sanctuary in Brisbane during their trip to Australia.
Up to 80 per cent of koalas in Australia are infected with the STI. The singers were told they could be carrying it after holding 3-year-old Kat.
Payne told The Sun: "I'm genuinely scared. This is worrying. I'd have never picked the thing up if I'd known."
The spread of chlamydia among koalas is a major threat to the future of the species, although transmission to humans is unlikely.
Payne and Louis Tomlinson recently bungee jumped off Auckland's Sky Tower
> One Direction: Rise of the world's biggest boyband - pictures
Take a look through our gallery of One Direction visiting a koala sanctuary below: